Tracing Saltwater Intrusion into Shallow Mangrove Aquifers Using Hydrogeochemical Analysis and Isotopes of Oxygen and Hydrogen

Abstract

Coastal aquifers that are hydraulically connected to the ocean suffer from the compromised of the wholesomeness of freshwater. The extent of the effect is largely dependent on the degree of aquifer stressing. Consequently, ratios of the stable isotopes of hydrogen (δ2H) and oxygen (δ18O) are integrated with ionic ratios of groundwater samples to assess the effect of intrusion on shallow aquifers in a low‐lying area of the western Niger Delta basin. Geochemical analysis shows sodium chloride (NaCl) water type, an indication that intrusion has a noteworthy effect on concentrations of groundwater by having values of total dissolved solids, electrical conductivity, and Cl− in excess of 1000, 500, and 250 mg L−1, respectively. Major ionic ratios of Na+/Cl− (0.06–1.76), Ca2+/Mg2+ (0.48–0.98), Cl−/HCO3− (148.77– 88), Cl−/HCO3− + SO42− (>1), and base exchange index (14.03 to 1.26) are largely indicative of intrusion. The δ18O and δ2H composition of groundwater and the brackish Forcados River samples reveal values that range from −4.2‰ to 1.08‰ and 0.18‰ to 3.8‰, respectively. The shallow groundwater samples are more depleted than the deep aquifers with δ18O values of −4.2‰ to −3.8‰ and δ2H of −20.99‰ to 18.51‰, respectively. The Cl−/δ18O ratios show the mixing of shallow aquifers with saltwater (SW) from the ocean.